VEINS AND LYMPHATICS IN TRAGULUS 219 



SUMMARY 



The development of the axial lymphatics in Tragulus presents 

 the following characteristics: 



1. Two distinct anlagen, one for the lymph sac and the other 

 for the systemic lymphatics. 



a. The lymph sac is derived from the venous system; passes 

 from the primary venous into the veno-lymphatic stage, at the 

 end of which period it loses all connection with the veins and 

 so enters upon its pre-lymphatic stage. Ultimately it establishes 

 a secondary connection with the venous system and so becomes 

 definitely lymphatic. 



b. The axial systemic lymphatics develop in three distinct por- 

 tions, namely the azygos, pre-azygos and post-azygos segments. 

 The exact histogenesis of these segments could not be determined, 

 but they convey the impression of plexiform channels arising 

 independently in the mesenchyme and then rapidly expanding 

 into discrete axial vessels. No connection between these seg- 

 ments and the veins was observed at any point. By confluence 

 the segments became integrated to form the axial systemic lym- 

 phatics. 



2. The final union of the two distinct anlagen determines the 

 completed axial lymphatic line. In this manner the lymph sac 

 becomes intermediary in establishing a communication between 

 the venous and peripheral lymphatic system, a mode of organi- 

 zation which resembles that of the cat and so, no doubt, the 

 general ground plan in mammals. 



Advancing from the stage of symmetrical channels, the venous 

 development is much concerned with modifications in the peri- 

 mesonephroic plexus. This vascular network, bearing intimate 

 relation to the mesonephros, is subsequently converted into such 

 distinct channels as the sub-cardinal and cardinal collateral veins 

 which have adapted themselves to the general line of axial growth. 



These facts seem to sustain the proposition that all definite 

 venous channels have their inception in a plexus and emerge 

 from this plexus as definite veins, under the influence of certain 

 hydro-dynamic factors, which are in the interest of most efficient 



